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authorRuslan Bilovol <ruslan.bilovol@gmail.com>2021-06-04 01:01:04 +0300
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2021-06-09 12:26:49 +0300
commite89bb4288378b85c82212b60dc98ecda6b3d3a70 (patch)
tree902329a9974240b3a8fe129d0e890e2dcd680cf1 /Documentation/usb/gadget-testing.rst
parent40c73b30546e759bedcec607fedc2d4be954508f (diff)
downloadlinux-e89bb4288378b85c82212b60dc98ecda6b3d3a70.tar.xz
usb: gadget: u_audio: add real feedback implementation
This adds interface between userspace and feedback endpoint to report real feedback frequency to the Host. Current implementation adds new userspace interface ALSA mixer control "Capture Pitch 1000000" (similar to aloop driver's "PCM Rate Shift 100000" mixer control) Value in PPM is chosen to have correction value agnostic of the actual HW rate, which the application is not necessarily dealing with, while still retaining a good enough precision to allow smooth clock correction on the playback side, if necessary. Similar to sound/usb/endpoint.c, a slow down is allowed up to 25%. This has no impact on the required bandwidth. Speedup correction has an impact on the bandwidth reserved for the isochronous endpoint. The default allowed speedup is 500ppm. This seems to be more than enough but, if necessary, this is configurable through a module parameter. The reserved bandwidth is rounded up to the next packet size. Usage of this new control is easy to implement in existing userspace tools like alsaloop from alsa-utils. Signed-off-by: Ruslan Bilovol <ruslan.bilovol@gmail.com> Signed-off-by: Jerome Brunet <jbrunet@baylibre.com> Link: https://lore.kernel.org/r/20210603220104.1216001-4-jbrunet@baylibre.com Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Diffstat (limited to 'Documentation/usb/gadget-testing.rst')
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diff --git a/Documentation/usb/gadget-testing.rst b/Documentation/usb/gadget-testing.rst
index f5a12667bd41..9d6276f82774 100644
--- a/Documentation/usb/gadget-testing.rst
+++ b/Documentation/usb/gadget-testing.rst
@@ -729,6 +729,7 @@ The uac2 function provides these attributes in its function directory:
c_srate capture sampling rate
c_ssize capture sample size (bytes)
c_sync capture synchronization type (async/adaptive)
+ fb_max maximum extra bandwidth in async mode
p_chmask playback channel mask
p_srate playback sampling rate
p_ssize playback sample size (bytes)